1. Field of the Invention
The present invention relates to a piezoelectric electroacoustic transducer such as a piezoelectric buzzer and a piezoelectric receiver.
2. Description of the Related Art
In electronic devices, electronic home appliances, portable telephones, and other such electronic devices, piezoelectric electroacoustic transducers have been widely used as a piezoelectric buzzer or piezoelectric receiver for producing an alarm sound or an operating sound. A piezoelectric electroacoustic transducer of this type generally includes a circular piezoelectric element bonded to one surface of a circular metallic plate to form a unimorph-type diaphragm, and the periphery of the metallic plate is mounted on a circular case using silicone rubber while an opening of the case is closed with a cover.
However, when using a circular diaphragm, there has been a problem of low production efficiency, low acoustic conversion efficiency, and moreover, difficulty in miniaturizing the piezoelectric electroacoustic transducer.
A piezoelectric electroacoustic transducer is described in Japanese Unexamined Patent Application Publication No. 2000-310990 as improving production efficiency and acoustic conversion efficiency, and can be miniaturized by using a rectangular diaphragm. The piezoelectric electroacoustic transducer includes a rectangular piezoelectric diaphragm, an insulating case having a bottom wall, four sidewalls, and a supporting member, which is disposed inside between two of the sidewalls opposing each other for supporting the diaphragm, the supporting member being provided with first and second conductive members formed therein for external connection, and a lid plate having holes formed thereon for releasing sound. The diaphragm is accommodated within the case, where two opposite sides of the diaphragm are fixed to the supporting member with an adhesive or an elastic sealant, while gaps between the remaining two sides of the diaphragm and the case are sealed with the elastic sealant, so that the diaphragm and the first and second conductive parts are electrically connected together with a conductive adhesive, and the lid plate is bonded on the open end of the sidewalls of the case.
As another method of fixing the piezoelectric diaphragm, there is also a method of fixing the four sides of the diaphragm to the supporting member with an adhesive or an elastic sealant.
In the diaphragm, the bending vibration may occur in a length-bending mode and the bending vibration in an area-bending mode, corresponding to a manner in which the diaphragm is supported. The former is a mode of bending-vibration in the plate-thickness direction with both ends in the longitudinal direction functioning as the supports where two sides of the diaphragm are fixed to the case, and the latter is a mode in which by fixing four sides of the diaphragm to the case, the entire area of the diaphragm flexurally vibrates in the plate-thickness direction with the four sides functioning as the supports such that the intersection of the diagonal lines has the maximum displacement.
However, the bending vibration in any of a conventional length-bending mode and area-bending mode has a defect in that the resonance frequency of the diaphragm is high and the sound pressure in the low frequency range cannot be increased. When increasing the size of the case and the diaphragm, the resonance frequency can be reduced in frequency. However, the piezoelectric electroacoustic transducer will be thereby increased in size.
Also, the conventional piezoelectric electroacoustic transducer has had a problem that when the diaphragm has a warp or winding, the resonance frequency is not stabilized, because two sides or four sides of the diaphragm are strongly restrained.